how to keep users logged in

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C#

ASP.NET

Hi everybody,
My project consist of creating a web-based search application that allows users to search for employees using a Sql database table named "Employee". I have created a page where the user enters search criteria, hit search button a grid view will display results, next to each row found there is a link that lead the user to another page containing detailed information about that Employee, this page is a DetailsView. the 3rd page consist of creating a new employee not found in database. so it also consist of DetailsView. I also have a login Page.
here is the catch: I want to allow everybody to access the search result page (Gridview) but only Admins (who exist in a sql DB table named Roles) have the right to modify data on the DetailsView Detailes info Page, I ve done this using the

DetailsView1.AutoGenerateEditButton = true;
                    DetailsView1.AutoGenerateDeleteButton = true;

but I want restrict regular users from accessing AddNewEmployee Page and Searchresult page with gridview on Edit mode, my login page looks like that:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Web.Security;
using System.Text;
using System.Security.Cryptography;
using System.IO;


namespace Phonebook
{
    public partial class LoginUser : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            
        }
       

        public string encryptQueryString(string strQueryString)
        {
            
            return BLL.Encryption64.Encrypt(strQueryString, "!#$a54?3");
        }


        protected void Login1_Authenticate(object sender, AuthenticateEventArgs e)
        {

            string username = Login1.UserName;
            string pwd = Login1.Password;
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString);
            conn.Open();
            string sqlUserName = "SELECT UserId, Password FROM Roles ";
            sqlUserName += " WHERE (UserId ='" + username + "')";
            sqlUserName += " AND (Password ='" + pwd + "')";
            sqlUserName += " AND (Admin = 'Y')";
            SqlCommand cmd = new SqlCommand(sqlUserName, conn);
            cmd.Parameters.AddWithValue("@UserId", username);
            cmd.Parameters.AddWithValue("@Password", pwd);

            string currentName;
            currentName = (string)cmd.ExecuteScalar();
            if (currentName != null)
            {
                //AddEmployee AdEmp = new AddEmployee();
                Session["UserAuthentication"] = username;
                Session.Timeout = 1;
                Response.Redirect("~/AddEmployee.aspx?search=" + encryptQueryString(username) + "");
            }
            else
            {
                Session["UserAuthentication"] = "";
            }
        }

I am using login control but not the ASP.NET configuration tool.
Is there any way I can create a login logic that does this? Thank you